1. Field of the Invention
The invention relates to dental crown forms and the method of installing dental crown forms.
2. Brief Description of the Prior Art
The installation of permanent crowns to reconstruct a tooth, particularly human teeth, is a commonplace procedure generally accomplished in a sequence of steps over a prolonged period of time. Initially, impressions of the tooth to be reconstructed may be made to establish relationships with adjacent teeth. Then the tooth is ground to remove damaged portions and to provide a shape or "stump" which is adapted to receive the crown jacket. Impressions or a mold is made of the shaped stump for transmittal to a dental laboratory where the permanent crown is molded. Prior to permanent fixation of the crown, a number of fittings may be required. In the meantime, it is necessary to protect the shaped tooth or stump from shock, further damage and exposure which could ultimately result in loss of the tooth. To accomplish this, a temporary crown or "crown form" is installed on the shaped tooth immediately. Desirably, the crown form is quickly installed, well fitting, durable, easily removed and replaced and completely protective of the shaped tooth.
Conventional crown forms come in several types. One type is a shell of thin metal such as aluminum or copper. They are affixed by lining with a fluid dental cement and forcing over the shaped tooth. As supplied, these metal crown forms generally do not correspond closely to the configuration of the tooth to be protected. For proper installation the shape of the form must be altered by the operator in an effort to fit the tooth. For example, if the crown form is too long, it must be trimmed along the cervical edge and festooned to correspond to the gum line. In addition, the open end of the metal crown form may have to be crimped inwardly to reduce its lateral dimension and to provide a configuration similar to that of the cervical end of a tooth. Such contouring is difficult to carry out without causing folds in the material of the crown form. In any event, the material of the crown never can be gathered in sufficiently to produce a close fit with the tooth. As a result, there is a projecting edge at the cervix of the crown form that is irritating to the gum tissue and uncomfortable to the patient.
Another difficulty of the metallic crown form resides in carrying out its installation. An excessive amount of dental cement is used to line the crown form and then it is forced over the shaped tooth. This expresses the excess cement through the cervical opening, under pressure. If the cement sets, additional steps are required to cut or grind away the excess cement adhering to the cervical line of the crown form. To overcome this, it has been suggested to place vent holes in the lingual or occlusal surfaces of the crown form. However, the cement expressed through vent holes positioned in those locations, upon setting, must also be removed as it is discomforting to the patient.
The metallic crown forms suffer other disadvantages, re; oxidation in oral fluids and their lack of color appeal. To meet these objections, celluloid crown forms were introduced. Although these forms presented a number of advantages over the metal crowns, they require cutting to obtain a good fit and are difficult to remove when removal is desired. These characteristics make use of the celluloid forms time consuming for the busy operator. The celluloid crown form also poses the same problem of expressing excess cement along the cervical edge, requiring removal, as described above in relation to the metal forms.
In recent years, synthetic polymeric resin (polycarbonate) forms have become commercially available. These forms are durable, but large inventories of shapes and sizes must be maintained since they are difficult to adapt to any given shape or size. Unless one is fortunate enough to find a form which is a perfect mate to the shaped tooth, extensive cutting is required to adapt the form. Much of the cutting requires grinding with a grindstone, which causes a "balling" of the plastic. Further, the commercially available polycarbonate crown forms suffer from the disadvantages of the prior crown forms, ie; the cervical edges must be trimmed, festooned and later trimmed of excess cement expressed through the cervical opening during insertion over the shaped tooth. Further, the commercially available polycarbonate crown forms generally fail to make mesial-distal tooth contact properly because they are not adjustable in this direction without extensive modification.
The crown forms of my invention are an improvement over the prior art crown forms in a number of respects. For example, they include a hinge structure so that they will accommodate teeth of variable dimensions. This is particularly valuable when fitting the crown to molar teeth and reduces the time required for preparing the crown form. Further, the new crown form of my invention may be cut with scissors when necessary to trim the cervical line. This eliminates the "balling" of the plastic caused by application of a grindstone. The unique positioning of a means for allowing the escape of excess dental cement from the interior of the crown form during installation facilitates the serial assembly, as in replacement of missing teeth, in a fixed prosthesis and serves to anchor single units. This is a particular advantage when installing a single crown form since most of the dental cements employed require at least 48 hours before full strength is achieved. During this setting period, initial mastication can drive the crown form further onto the shaped tooth. If the occlusal surface falls beneath the level of the adjacent teeth, a hypo-occlusion occurs. Therefore, no pressure is received by the temporarily crowned tooth during occlusion. The tooth may then erupt from its original position or the opposing tooth will drift toward the crown tooth. Later when the permanent crown is applied, it will not occupy the same position relative to the opposing tooth that it was originally made for, causing a hyper-occlusion. This problem is less likely to occur when the single crown form, as installed initially, is anchored or bridged to the adjacent teeth.
The novel crown forms of the invention provide the means for the method of the invention. The method of the invention as described hereafter, has advantages of speed, economy of materials and results in a better fitting, more comfortable prosthesis.